In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, multi-antenna techniques have the potential to increase the data rates and reliability of a wireless communications network. The transmitter and the receiver are equipped both being provided with multiple antennas results in a multiple-input multiple-output (MIMO) communication channel.
The Long Term Evolution (LTE) standard is currently evolving with enhanced MIMO support. One component in LTE is the support of MIMO antenna deployments and MIMO related techniques. Currently LTE-Advanced supports an 8-layer spatial multiplexing mode for 8 transmitter antennas with channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of a transmission point 120 configured for such a spatial multiplexing operation is provided in FIG. 1.
As seen in FIG. 1, an information carrying symbol vectors of rank r (as represented by Layer 1, Layer 2, . . . , Layer r) is multiplied by an NT-by-r precoder matrix W, which serves to distribute the transmit energy in a subspace of an NT-dimensional vector space (corresponding to NT antenna ports) via respective inverse fast Fourier transform (IFFT) blocks. The precoder matrix could be selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r could be adapted to suit the current channel properties.
LTE uses orthogonal frequency-division multiplexing (OFDM) in the downlink (i.e., from the transmission point in the communications network to served wireless devices) and discrete Fourier transform (DFT) precoded OFDM in the uplink (i.e., from the wireless devices to the transmission point) and hence the received NR-by-1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en where en is a noise/interference vector obtained as realizations of a random process. The precoder W can be a wideband precoder, which is constant over frequency, or be a frequency selective precoder.
The precoder matrix W is can be selected to match the characteristics of the NR-by-NT MIMO channel matrix Hn, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the wireless device. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the wireless device, the inter-layer interference is reduced.
In closed-loop precoding for the LTE downlink, the wireless device transmits, based on channel measurements in the forward link (downlink), recommendations to the network node controlling the transmission point of a suitable precoder to use. The network node configures the wireless device to provide feedback according to the transmission mode used by the wireless device, and may transmit channel state information—reference signals (CSI-RS) and configure the wireless device to use measurements of CSI-RS to feed back recommended precoding matrices that the wireless device selects from a codebook. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband. This is an example of the more general case of channel state information (CSI) feedback, which also encompasses feeding back other information than recommended precoders to assist the network node in subsequent transmissions to the wireless device. Such other information may include channel quality indicators (CQIs) as well as transmission rank indicator (RI).
Given the CSI feedback from the wireless device, the network node determines the transmission parameters it wishes to use to transmit to the wireless device, including the precoding matrix, transmission rank, and modulation and coding scheme (MCS). These transmission parameters may differ from the recommendations the wireless device makes. Therefore a rank indicator and MCS may be signaled in downlink control information (DCI), and the precoding matrix can be signaled in DCI or the network node could transmit a demodulation reference signal from which the equivalent channel can be measured. The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder W. For efficient performance, a transmission rank that matches the channel properties should be selected.
With multi-user MIMO, two or more wireless devices served by the same transmission point are co-scheduled on the same time-frequency resource. That is, two or more independent data streams are transmitted to different wireless devices at the same time, and the spatial domain is used to separate the respective streams. By transmitting several streams simultaneously, the capacity of the communications network can be increased. This could reduce the signal-to-interference-plus-noise ratio (SINR) per data stream since the power has to be shared between data streams and the transmissions of the data streams could cause interference to each-other.
When increasing the antenna array size, the increased beamforming gain could lead to higher SINR. The multiplexing gain increases linearly with the number of multiplexed wireless devices. Thus, as the user throughput depends only logarithmically on the SINR (for large SINRs), it could be beneficial to trade the gains in SINR for the multiplexing gain.
Accurate CSI is required in order for the network node to perform appropriate nullforming between co-scheduled wireless devices (i.e. in order to achieve that transmissions to co-scheduled wireless devices do not mutually interfere with each other). In the current LTE Rel. 13 standard, no special CSI feedback mode for multi-user MIMO (MU-MIMO) exists and thus, feedback-based MU-MIMO scheduling and precoder construction has to be based on the existing CSI reporting designed for single-user MIMO (that is, a PMI indicating a DFT-based precoder, a RI and a CQI). Thus, pairing of wireless devices for MU-MIMO and the corresponding link adaptation has to be based on the reported PMI and may for example be derived by calculating the orthogonality of the reported precoders for each user. However, PMI reports are not reliable in multi-user MIMO scenarios where two or more wireless devices are co-scheduled.
Hence, there is still a need for improved mechanisms for co-scheduling wireless devices in a communications network.